1. Field of the Invention
This invention relates in general to the field of digital signal processing in digital communication. More particularly, the invention relates to a reciprocal phase calculation apparatus and method thereof in a digital communication system.
2. Description of Related Art
The telephone networks currently in place were originally designed for transmission of electrical signals carrying human speech. Since human speech is generally confined within a band ranging from 0 Hertz to 3,400 Hertz, telephone networks were designed to provide telephone lines to each user which were capable of handling frequencies within this range. Today, these same telephone lines, which connect a service user to a central office, are in place, permitting communication of only voice data or analog modem transmissions of not more than 56,000 bits per second. However, connections between central offices of telephone networks are provided by high-bandwidth fiber optic transmission facilities in nearly every telephone network worldwide.
Because the local telephone lines which connect an end user to a central office are only capable of handling frequencies of up to 3,400 Hertz, communication equipment utilizing these lines, such as dial modems or fax modems, have been accordingly limited in bandwidth. Despite the presence of high bandwidth fiber optic lines between central offices, users remain limited in the bandwidth available to them because the local lines serve as a bottleneck. New technologies, such as the Internet or video conferencing, demand that the bottleneck be removed.
Digital Subscriber Line (DSL) technologies are capable of removing the bottleneck. DSL pennits a user to communicate over the existing telephone lines at a rate of tens of millions of bits per second. In order to utilize DSL, a site must be equipped with a transceiver (a DSL modem) which communicates, via the existing telephone lines, with another transceiver located at the central office of the network access provider, generally the local telephone company.
Generally, the DSL communication systems are symmetric modems typically include symmetric transmit and receive paths. An example of such a system is shown in FIG. 1, where a sample rate conversion is employed to transfer data from a conversion clock CLK2 of the analog-to-digital (A/D) and the digital-to-analog (D/A) converters to a system clock CLK1 of the digital signal processing (DSP) core. In the transmit (TX) path of such a symmetric communication system, data is sent from the DSP core, which operates on the first clock domain (CLK1), to a frequency conversion system which operates on the second clock domain (CLK2). Likewise, in the receive (RX) path, data from the frequency conversion system which operates on the second clock domain (CLK2) is passed to the DSP core which operates on the first clock domain (CLK1). The sample rate conversion in the transmit (TX) path can be accomplished through the use of a numerically controlled oscillator (NCO) with a phase word WTX=f1/f2, wherein f1 represents the frequency of CLK1, and f2 represents the frequency of CLK2. Similarly, the sample rate conversion in the receive (RX) path can be accomplished through the use of an NCO with a reciprocal phase word WRX=f2/f1=1/WTX.
However, the problem of using a reciprocal phase word is that finite precision effects within the implementation of the system can cause WTXxe2x89xa01/WRX which will result in an accumulated phase error (i.e. frequency drift) at a receiver relative to a transmitter. For example, if WTX=3, then WRX=0.333 (or 0.3333 . . .) which cannot be within the receiver. If this accumulated phase error is not compensated for by a specialized circuit such as a phase locked loop (PLL), the sample frequency drift between the transmitter and the receiver will eventually cause a system failure. A generic communication system which employs an TX NCO and an RX NCO is shown to the receive rate converter. R can be any suitable integer depending on the system. As an example, if there is no decimation occurring prior to the receive rate converter, R is equal to one (1). In such a system, the phase word WRX for the RX NCO is computed from the phase word WTX for the transmit NCO. If the phase word 1/WTX cannot be to avoid system failure.
Accordingly, there is a need for an improved reciprocal phase calculation such that it provides for arbitrary sample frequency conversion from CLK2 to CLK1 while preventing and eliminating accumulated phase error for the frequency conversion.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a reciprocal phase calculation apparatus and a method of calculating reciprocal phase relations between a transmit rate converter and a receive rate converter of a digital communication system.
The present invention solves the above-described problems by providing a reciprocal phase calculation apparatus which provides for arbitrary sample frequency conversion from CLK2 to CLK1 and computes the receive sample phase from the transmit (TX) NCO value. The present invention eliminates the receive (RX) NCO and any accumulated errors that result from representing WTX=1/WRX with finite precision.
In one embodiment of the present invention, a receive rate converter phase calculation apparatus comprises a transmit (TX) numerical controlled oscillator (NCO) converting a sample rate (or frequency) of transmit data from a first clock to a second clock and generating a modulo signal indicating a residual phase; and a receive phase calculator receiving the modulo signal and generating a receive sample phase such that a sample rate (or frequency) of receive data is converted from the second clock to the first clock by interpolating the receive sample phase between two adjacent signals of the receive data.
Other embodiments of the converter in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that the TX NCO includes: an accumulator which accumulates a phase word (or a baud rate) at each tick of the second clock; and a modulo indicator which indicates the residual phase in a cycle of the second clock after a cycle of the first clock.
Another aspect of the present invention is that the receive phase calculator includes a first multiplier for multiplying the modulo signal and an inversion of the baud rate; an adder for adding a value which represents a number of decimated clock periods of the first clock and a minus value outputted from the first multiplier; and a second multiplier for multiplying an inversion of an integer which represents the number of decimated clock periods of the first clock and a value outputted from the adder.
The present invention also discloses a symmetric communication system that processes data using one clock domain (CLK1) and transmits and receives data in a conversion system utilizing a second clock domain (CLK2). The present invention relates to a receive rate converter phase calculation apparatus and method thereof in the symmetric communication system. In one embodiment of the present invention, the communication system comprises a timing circuit which generates phase conversion information from a transmitter to transfer data from the first clock domain to the second clock domain; and a receive phase calculation circuit that utilizes the phase conversion information from the transmitter to transfer data from the second clock domain to the first clock domain.
The present invention further discloses a method of utilizing a phase conversion information of a transmitter of a digital communication system during its conversion from a first clock domain to a second clock domain, to transfer data from the second clock domain to the first clock domain at a receiver of the digital communication system. The method comprises converting a sample rate (or frequency) of transmit data from the first clock to the second clock; generating a modulo signal indicating a residual phase in a cycle of the second clock after a cycle of the first clock; generating a receive sample phase; and interpolating the receive sample phase between two adjacent signals of receive data for conversion of a sample rate (or frequency) of the receive data from the second clock to the first clock at the receiver.
These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention.